Switched mode power supplies (SMPS) are useful for a variety of situations. Typical SMPS are configured to have an upper threshold on the output voltage. In some applications, a boost supply is desired where the output voltage is required to be as much as twice the input voltage. Slope compensation may be used to provide stability to the current control loop of the SMPS. Known slope compensation techniques include adding a positive slope to a current sense voltage or adding a negative slope to a reference voltage. When the current sense voltage exceeds the reference voltage, the switch is turned off.
The magnitude of the slope required for dampening the transient step response in a SMPS is well studied. For a boost supply configuration, a function of the slope of an inductor current during the off time of the switch provides the appropriate slope. Including such a compensation slope is relatively straightforward in a closed loop configuration utilizing negative feedback because the average inductor current is equal to the load current. In open loop implementations, however, it is challenging to set the reference voltage so that the compensated reference voltage and the current sense voltage intersect when the conductor peak current is at a desired value. Known techniques require knowledge of the input voltage and the output voltage, among other parameters. In some situations, the input voltage tends to be ill-defined, which is the case when the input voltage is based on a typical automotive vehicle battery. The peak current may vary significantly with such an input voltage source.
It is challenging to control the reference voltage in a manner that provides circuit stability and keeps the inductor current within desired limits. If the inductor current is not controlled adequately, more expensive components are required.